1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) device material and an organic electroluminescence (EL) device using the same.
2. Description of Related Art
An organic EL device, which includes an organic thin-film layer (in which an emitting layer is included) between an anode and a cathode, has been known to emit light using exciton energy generated by recombination of holes and electrons that have been injected into the emitting layer (see e.g., Patent Literature 1: WO2003-080760, Patent Literature 2: JP-A-2006-188493, Patent Literature 3: JP-A-2006-199679, Patent Literature 4: JP-A-2004-178895, and Patent Literature 5: JP-A-2000-169448).
Such an organic EL device, which has the advantages as a self-emitting device, is expected to serve as an emitting device excellent in luminous efficiency, image quality, power consumption and thin design.
In forming the emitting layer, a doping method, according to which an emitting material (dopant) is doped to a host, has been known as a usable method.
The emitting layer formed by the doping method can efficiently generate excitons from electric charges injected into the host. With the exciton energy generated by the excitons being transferred to the dopant, the dopant can emit light with high efficiency.
Recently, in order to upgrade an organic EL device, a further study on a doping method has been made to seek favorable host materials.
For instance, Patent Literatures 1 to 5 disclose inventions related to such host materials. Patent Literatures 1 to 5 disclose compounds, such as below-listed Compounds I to VI, in which a carbazolyl group and a nitrogen-containing aromatic ring are contained in the same molecule.
Compound I disclosed in Patent Literature 1 has a structure in which two carbazole skeletons are bonded to a benzene ring in meta positions (3,5-positions) and an electron-deficient nitrogen-containing hetero aromatic ring structure. A carbazole skeleton (a representative example of which is polyvinylcarbazole) has been known through the ages as a main skeleton for a hole transporting material. In contrast, an electron-deficient nitrogen-containing hetero aromatic ring structure has been known as a structure with a high electron transporting capability. Compound I disclosed in Patent Literature 1, in which a hole transporting skeleton and an electron transporting skeleton are combined, is a material aimed at a balanced charge transport.
In Compound II disclosed in Patent Literature 1, two structures in each of which two carbazole skeletons are bonded to a benzene ring in meta positions (3,5-positions) are symmetrically disposed relative to a pyridine ring as an electron-deficient nitrogen-containing hetero aromatic ring.

However, Compound I is structured such that the carbazole skeletons are bonded to the benzene ring in meta positions and, thus, the carbazolyl groups are bent rightward and leftward relative to the bond axis between the pyrimidine ring and the benzene ring (two conjugated aromatic rings), hampering overlap of the carbazole skeletons between molecules. In view of this, since the hole transporting capability is insufficient and positions of electron recombination tend to be shifted toward the anode, it may not be possible to provide favorable luminescence property and life property.
Compound II is likewise structured such that carbazole skeletons are bonded to a benzene ring in meta positions and, thus, the carbazolyl groups are bent rightward and leftward relative to the bond axis between a pyridine ring and the benzene ring (two conjugated aromatic rings), so that the hole transporting capability is insufficient for the same reason.
This is also applicable to Compound III disclosed in Patent Literature 2. Specifically, since carbazolyl groups are bent relative to the bond axis between a pyrimidine ring and a benzene ring to which carbazole skeletons are bonded, the hole transporting capability is insufficient for the above reason.
In Compound IV disclosed in Patent Literature 3, carbazolyl groups are bonded to a benzene ring in 2,5 positions, which are sterically-congested bond positions relative to the bond axis between a pyrimidine ring and a benzene ring to which the carbazole skeletons are bonded (two conjugated aromatic rings). In view of this, it is predicted that overlap of the carbazole skeletons between the molecules is further hampered and, thus, it may not be possible to provide a sufficient hole transporting capability.
In contrast, Compound V disclosed in Patent Literature 4 is structured such that each of three carbazolyl groups is disposed on an extension (4-position, para position) of the bond axis between a benzene ring to which the carbazolyl group is bonded and a pyrimidine ring (two conjugated aromatic rings). Compound VI disclosed in Patent Literature 5 is structured such that each of three carbazolyl groups is disposed on an extension (4-position, para position) of the bond axis between a benzene ring to which the carbazolyl group is bonded and a pyridine ring (two conjugated aromatic rings). In such a structure, an increased overlap of the carbazolyl groups between the molecules is expected to improve the hole transporting capability. In fact, 4,4′-N,N′-dicarbazolyl-biphenyl (CBP) structured such that two carbazolylphenyl groups are bonded in para positions has been known to have a significantly high hole transporting capability. However, all the carbazolyl groups are disposed in 4-positions (para positions), which makes the hole transporting capability extremely high. In view of this, it may be difficult to provide an optimal carrier balance in an organic EL device.